Which device is used to control the direction of a DC motor?

The device used to control the direction of a DC motor is a polarity switch. This device effectively changes the polarity of the voltage applied to the motor terminals, which in turn reverses the direction of the motor's rotation. By swapping the connections of the positive and negative terminals, the flow of current through the motor windings is reversed, allowing the motor to turn in the opposite direction. This method is specifically suited for DC motors, which rely on the direction of current flow to determine rotational direction.

Other options, while they may have their own functions, do not specifically serve the purpose of reversing a DC motor's direction. For instance, a relay is primarily used for switching circuits and controlling power to devices rather than directly changing the flow of current for directional control. A switch can turn a circuit on or off but does not inherently reverse the polarity. A frequency drive, designed for AC motors, controls motor speed and torque by varying the frequency and voltage supplied to the motor, but it does not directly apply to the directional control of a DC motor. Hence, the polarity switch is the most appropriate choice for determining the direction of a DC motor.